1. Field of the Invention
The present invention relates generally to improvements in optical communication systems. Note particularly, the present invention relates to techniques for phase conjugating and/or frequency converting optical signals in an optical communication system using four-photon mixing.
2. Description of Prior Art
Optical communication typically involves transmitting high bit rate digital data over silica glass fiber by modulating a laser or other optical signal source. Although glass fiber has a very broad bandwidth, on the order of 40,000 THz, the maximum data rate which can be transmitted across a given length of fiber is limited by chromatic dispersion and nonlinearities within the fiber. Dispersion and nonlinearities spread an optical signal in time and frequency, respectively, as it propagates through the fiber. Chromatic dispersion, often simply called dispersion, refers to a phenomenon in which the speed of an optical signal through the fiber varies as a function of the optical signal frequency or wavelength. Chromatic dispersion is generally considered linear to the first order as a function of wavelength, and second order dispersion is therefore approximated to be zero. Nonlinearities, on the other hand, involve changes in the propagation speed of an optical signal as a function of the signal amplitude or intensity. One commonly encountered nonlinearity is the Kerr effect, in which the fiber index of refraction increases with increasing optical signal amplitude. For systems transmitting at a given data rate, these dispersion and nonlinear effects limit the achievable non-regenerated transmission distance. As a result, for long-haul optical communication links, it is necessary to either control, compensate or suppress dispersion and nonlinearity, or use regenerative repeaters along the fiber transmission path.
One of the known techniques for compensating first order chromatic dispersion effects in fiber uses midsystem optical phase conjugation to balance the first-order dispersion occurring in the first half of a fiber transmission span with that occurring in the second half of the span. Because phase conjugating a given signal reverses its phase, mid-span conjugation allows the effects of first order dispersion in each half of the span to cancel out. See A. Yariv, D. Fekete and D. Pepper, "Compensation for channel dispersion by nonlinear optical phase conjugation", Optics Letters, vol. 4, pp. 52-54, 1979. By counteracting first order linear dispersive effects in this manner, midsystem optical phase conjugation has extended the bit rate distance product achievable over the anomalous dispersion fiber which make up much of the world's existing fiber communication channels. See A. Gnauck, R. Jopson and R. Derosier, "10 Gb/s 360 km Transmission over Dispersive Fiber Using Midsystem Spectral Inversion", IEEE Photonics Technology Letters, vol.5, no.6, Jun. 1993. It can be seen, therefore, that in optical communication systems it is often desirable to produce the phase conjugate of an optical signal.
Phase conjugation of optical signals is typically performed using four-photon mixing, also commonly referred to as four-wave mixing. Four-photon mixing is a nonlinear optical process which produces mixing products by mixing an input optical communication signal with one or more higher power optical signals, or pumps, in a nonlinear mixing medium such as a semiconductor laser, a semiconductor laser amplifier or a length of dispersion-shifted optical fiber. However, efficiency of the four-photon mixing process depends upon the relative polarizations of the optical signal and the pump. Since the signal polarization in a fiber-optic communications link varies randomly with time, or can become depolarized, it is difficult to maintain optimal efficiency in the four-photon mixing process by controlling the input signal polarization. Thus, the efficiency of the four-photon mixing process will vary randomly. Since failure to maintain proper polarization alignment between the signal and the pump will result in a decrease in mixing product signal power, mixer output power will also vary randomly. In the case of four-photon mixing to obtain a phase conjugate, the advantages of optical phase conjugation would often be more than offset by such variation in conjugated signal power.
A recently demonstrated experimental technique attempts to alleviate polarization sensitivity of the four-photon mixing process by using a polarization beam splitter and a fiber loop to produce and mix orthogonally-polarized versions of both the incoming optical signal and the pump. See T. Hasegawa et al., "Multi-Channel Frequency Conversion Over 1 THz Using Fiber Four-Wave Mixing", Post Deadline Digest of the Optical Amplifiers and their Applications Conference, paper PD7, Jul. 4-6, 1993, Yokohama, Japan. Although the Hasegawa fiber loop four-photon mixing technique apparently reduces the sensitivity of the mixing process efficiency to incoming signal polarization, it suffers from a number of significant drawbacks. For example, a polarization controller is required in the fiber loop in order to effectuate the proper recombination of the different polarizations of the mixing products. This leads to additional hardware costs both for the polarization controller itself as well as for any additional devices required to appropriately adjust the polarization controller. Furthermore, the fiber loop requires relatively long lengths of dispersion-shifted or non-dispersive fiber to serve as a nonlinear medium for four-photon mixing. Thus the technique is not compact and cannot be implemented in a commercially advantageous form such as a photonic integrated circuit.
As is apparent from the above, a need exists for a polarization-insensitive optical mixing technique which maintains the optimal relationship between signal and pump polarization so as to produce maximum frequency converted and/or conjugated signal output power regardless of the time-dependent state of the input signal polarization. Maximum benefit will thereby be obtained from dispersion compensation techniques utilizing optical phase conjugation. The optical mixer should not require a polarization controller or other manual or automated adjustment hardware. Furthermore, the optical four-photon mixer should be useful with any nonlinear mixing device, and therefore suitable for implementation in the form of a photonic integrated circuit.